Investigating the Effect of Succinate On PQQ Operon Genes and mGDH Activity

Abstract

In soil, phosphorus is the second most abundant element present, but most of it is present in insoluble form and hence is unavailable for the plants to take up. Glucose is the most utilized carbon source for organic acid mediated phosphate solubilization. Acinetobacter sp. SK2, a mineral phosphate solubilizing bacteria isolated from the rhizosphere of Vigna radiata (Bharwad and Rajkumar., 2020) solubilizes 682 µg ml-1 tricalcium phosphate (TCP) and 86 µg ml-1 rock phosphate (RP) on production of gluconate leading to decrease in the pH. Acinetobacter sp. SK2 also produced IAA (117 μg ml-1), siderophore (87% units), HCN, ammonia and solubilized minerals of Zinc and Potassium. The enzyme responsible for gluconate production is periplasmic glucose dehydrogenase (mGDH) and soluble glucose dehydrogenase (sGDH). The PQQ co factor required for glucose dehydrogenase activity oxidizes glucose and converts it into gluconate decreasing the surrounding pH and ultimately leading to phosphate solubilization. The present study aims to determine the effect of succinate on pqq operon genes. Genes corresponding to the pqq operon were retrieved from whole genome sequence of Acinetobacter pittii PHEA2 and compared with pqq genes found in different members of Pseudomonas and Acinetobacter genera. Differential expression analysis of pqq genes in cells grown under CCR and non-CCR conditions revealed that the expression of these genes was considerably down regulated under repression conditions (glucose + succinate). This indicated that genes responsible for PQQ synthesis were also under repression during CCR condition. This suggests that MPS phenotype is under both direct and indirect repression when preferred (succinate) and non-preferred (glucose) carbon source are present together. Further we aimed at determining whether the synthesized PQQ is also released outside the cells, while growing under non-CCR conditions.